In “Chemistry in Microstructured Reactors,” Ang. Chem. Int. Ed. 2004, 43, 406-466 [: Applied Chemistry, International Edition], K. Jähnisch et al. have demonstrated the advantages that microstructured components have in chemical reactions and for analytical purposes. This has led to an increase in the importance that such systems have for chemical synthesis and analysis. In comparison to conventional reactors, these microstructures have a large surface area/volume ratio, which has a positive influence on the transference of heat as well as the process of the transportation of matter (see also: O. Wörz et al. “Micro-reactors—A New Efficient Tool for Reactor Development,” Chem. Eng. Technol. 2001, 24, 138-142).
Many known reactions have been carried out in microstructure reactors, including many catalytic reactions. For these, it is unimportant whether the reactions are liquid phase, gas phase or gas-liquid phase reactions. In order to take advantage of the potential activity of the catalyzer, the catalytic material is integrated in microstructured systems with various geometric forms. In the simplest case, the reaction material used for the construction of the micro-reactor consists in itself of the catalytically active substance (see also: M. Ficthner, “Microstructured Rhodium Catalysts for the Partial Oxidation of Methane to Syngas under Pressure,” Ind. Eng. Chem. Res. 2001, 40, 3475-3483). This means however that the catalytic surface is limited to the walls of the reactor. This disadvantage is partially resolved by means of optimized catalyzer/carrier systems. For the most part, current micro-structure reactors contain small particles or powder, which are incorporated in a channel.
Catalyzer filaments, wires and membranes are also used however (see also: G. Veser, “Experimental and Theoretical Investigation of H2 Oxidation in a High-Temperature Catalytic Microreactor,” Chem. Eng. Sci. 2001, 56, 1265-1273). Metallic nanostructures, particularly those from transition metals, are known in heterogenic catalysis due to their high ratio of surface area/mass, resulting in lower production costs (see also: R. Narayanan et al. “Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability,” J. Chem. Phys. B, 2005, 109, 12633-12676).
Originally, research was concentrated on the examination of isotropic metal particles, and as a result, their catalytic characteristics have been studied at length. At present, however, many one-dimensional nanostructures have been analyzed regarding their use in heterogenic catalysis. The stabilization of these is a major problem. The incorporation of nanostructures on a carrier or storage of them in porous matter such as, e.g. Nafion is known from Z. Chen et al. “Supportless Pt and PtPd Nanotubes as Electrocatalysts for Oxygen-Reduction Reactions,” Ang. Chem. 2007, 119, p. 4138-4141, which leads however directly to a decrease in the utilizable catalyzer surface area. Furthermore, it must be noted that the catalytic activity is dependent on the distribution of the catalyzer material due to the diffusion processes. Accordingly, the nanoparticles significantly increase the surface area/volume ratio, but long-term stability of such reactors is relatively limited due to the following:
1. Loss of contact between nanoparticles due to corrosion of the carrier.
2. Dissolving and renewed deposition or Ostwald ripening.
3. Aggregation of the nanoparticles in order to minimize the surface energy.
4. Dissolving of the nanoparticles and migration of the dissolvable ions.
Parallel wire and tube structures have already been used as glucose sensors (J. H. Yuan et al., “Highly Ordered Platinum-Nanotubule Arrays for Amperometric Glucose Sensing,” Adv. Funct. Mater. 2005, 15, 803), as electrocatalysts, for example, in alcohol oxidation (H. Wang et al., “Pd Nanowire Arrays as Electrocatalysts for Ethanol Electrooxidation,” Electrochem. Commun. 2007, 9, 1212-1216) and for hydrogen peroxide reduction (H. M. Zhang et al., “Novel Electrocatalytic Activity in Layered Ni—Cu Nanowire Arrays,” Chem. Commun. 2003, 3022). In these cases however, the nanostructures are freestanding, such that the arrangement is open and instable. Nielsch et al. have reported in “Uniform Nickel Deposition into Ordered Alumina Pores by Pulsed Electrodeposition,” Adv. Mater. 2000, 12, 582-586, that pulsed deposition is used for deposition of thin metallic foils.
All in all, there is still a great deal of potential for innovation in the field of nanotechnology.